The invention relates to a hip joint prosthesis having a metallic prosthesis shell with a cavity insert having a spherical recess and with an intermediate element which is made of plastics and arranged between the prosthesis shell and the cavity insert.
A hip Joint prothesis having three spherical calottes which lie one inside the other is known from the utility model FR 2 715 828.
The spherical outer calotte consists of a bio-compatible, rigid material such as INOX-steel or titanium, and is provided, on the outside, with radial depressions for securing in the articular cavity of the pelvic bone.
The spherical inner calotte consists of a hard, bio-compatible material, such as aluminium oxide ceramic, for example, or of a similar material, having a low coefficient of friction in relation to the material of which the head of the femur element, which is connected thereto, consists.
The spherical intermediate calotte consists of a semi-rigid material such as plastics, in particular polyethylene.
A problem with intermediate calottes or intermediate elements made of plastics is that, as a result of high abrasion, they can trigger osteolysis if there is no form-locking connection between the sliding shell and the plastics coating. On the other hand, a form-locking connection can be achieved by melting the plastics coating, as described in EP 0 554 214 A1. This can, however, lead to destruction of the molecular structure and consequently can trigger wear.
The underlying object of the invention is to improve a hip joint prosthesis according to the preamble of claim 1, in such a way that the danger of osteolysis is eliminated to a considerable extent.
This object is achieved in accordance with the invention by the characterising features of claim 1.
As a result of the intermediate element consisting of an upper portion and a lower portion, it being possible for upper portion and lower portion to be connected to each other, and the cavity insert being retained between upper portion and lower portion, the wear of the intermediate element is avoided and consequently the danger of osteolysis is reduced.
In a preferred embodiment, the cavity insert is covered on all sides, with the exception of its spherical recess, by the intermediate element, i.e. by the upper portion and lower portion, so that there are no free places at which abrasion can occur.
The connection of the upper portion to the lower portion can be carried out in a whole variety of ways. Screw connections by way of a screw thread, snap connections, pinning, gluing or welding have proven advantageous.
The cavity insert can be produced from metal or ceramics. This invention is favorable suitable for all ceramic cavity inserts.
In an advantageous manner, there are provided in the cavity insert and in the adjacent lower portion, opposing openings in which are placed inserts for protecting against rotation. As a result of this, the cavity insert sits in the intermediate element in a manner such that it is fixed in its position. The inserts can be, for example, pins, slotting blocks and feather-key systems respectively, or wedges and can be made, for example, from metal or plastics. For improved fixing, the inserts advantageously fill the openings completely.
Further features of the invention result from the Figures, which are described in the following.